Relativity and gravitons

[corvus_corax]

According to Einstein, gravity is really nothing more than a body of mass (such as the sun, the planet mars, or an entire galaxy) warping the spacetime around it.
And his model seems to be borne out to this day, with no appearances of "Einstein was wrong" that I'm aware of.
And quite frankly, I like that explanation (even though I over simplified it for this thread)

My question is, how does the proposed (and undemonstrated) "graviton" fit into this? Or does it at all?
From what I understand, the graviton is the "transmitter" or "communicator" of an actual force (in this case, gravity).
If that is so, then why does the warping/bending of spacetime by a mass body need a communicator or transmitter particle?
I realize the "rubber sheet" analogy regarding relativity is oversimplified, but it works (even S Hawking used it, very well I might add), but why does a warping of the 4 dimensions "need" a "communicator" particle such as the graviton?

1- Is the graviton necessary?
2- Is the graviton in opposition to relativity? (yes or no, please explain why)
3- If the graviton is "needed" for our understanding of particle physics, then is this specific area of Relativity incorrect?



Thanks in advance my fellow forumites

 

[Ectezus]

I too find this subject interesting. As far as I know the space fabric you often see in video's is just an analogy to explain the concept of gravity.
Personally I do not think it actually works that way directly but I'm no expert on the subject.

All calculations using the model however are extremely accurate but I'm more inclined to think it's actually some sort of 'pulling towards force' rather than a 'slope for things to fall down'. It's kinda like explaining gravity with gravity because why would those objects fall down the slope (downwards) in the first place... It makes sense in our human perspective though.

So as to your "why does a warping of the 4 dimensions "need" a "communicator" particle such as the graviton?" question. I think it's the other way around. The graviton particle is doing all the work and it needs a model/analogy of warping the 4 dimensions for us humans to understand the concept.

Since the graviton praticle is still hypothetical I'm not sure if you can get a 100% clear answer but I too would love to hear more from someone with more knowledge about the subject.

In the meantime, this video might interest you:
Garrett Lisi on his theory of everything | Video on TED.com

I watched it quite some time ago so I don't remember everything about it but it's about a model explaining the relation of all the particles, including gravity and that they all interact with eachother.

Not 100% sure about the correctness of it, but once again, a model with such a beautiful 'complex simplicity' I'm inclined to think it "simply must be true".
And yes, I'm fully aware that such unsupported feelings are the cause for a lot of wrong idea's (like... the God hypothesis ) so I don't consider it the truth unless proven.
Watch it yourself and tell me what you think and if it answers some of your questions.


- Ectezus

 

[corvus_corax]

I too find this subject interesting. As far as I know the space fabric you often see in video's is just an analogy to explain the concept of gravity.
Personally I do not think it actually works that way directly but I'm no expert on the subject.

All calculations using the model however are extremely accurate but I'm more inclined to think it's actually some sort of 'pulling towards force' rather than a 'slope for things to fall down'. It's kinda like explaining gravity with gravity because why would those objects fall down the slope (downwards) in the first place... It makes sense in our human perspective though.

So as to your "why does a warping of the 4 dimensions "need" a "communicator" particle such as the graviton?" question. I think it's the other way around. The graviton particle is doing all the work and it needs a model/analogy of warping the 4 dimensions for us humans to understand the concept.

Since the graviton praticle is still hypothetical I'm not sure if you can get a 100% clear answer but I too would love to hear more from someone with more knowledge about the subject.

In the meantime, this video might interest you:
Garrett Lisi on his theory of everything | Video on TED.com

I watched it quite some time ago so I don't remember everything about it but it's about a model explaining the relation of all the particles, including gravity and that they all interact with eachother.

Not 100% sure about the correctness of it, but once again, a model with such a beautiful 'complex simplicity' I'm inclined to think it "simply must be true".
And yes, I'm fully aware that such unsupported feelings are the cause for a lot of wrong idea's (like... the God hypothesis ) so I don't consider it the truth unless proven.
Watch it yourself and tell me what you think and if it answers some of your questions.


- Ectezus

Thank you for your explanation.
However, IF I understand it correctly, Relativity is not a model/analogy. IF I'm understanding it correctly it actually states (both mathematically and in "normal language") that objects with mass ACTUALLY "warp" space "slope of things falling down" (so to speak).
Quite frankly both Relativity theories (that I know of) are inviolable, and relativity explains gravity (as I understand it) via "path of least resistance" as opposed to "force communicating particles".


Jeez, I hope that made sense.
Again, thank you (and I'll watch the vid you posted).
I have to wonder though, where is WiccanChild and Arunma? (amongst others)


Is my question below recognition? ~sigh~

 

[sfs]

Thank you for your explanation.
However, IF I understand it correctly, Relativity is not a model/analogy. IF I'm understanding it correctly it actually states (both mathematically and in "normal language") that objects with mass ACTUALLY "warp" space "slope of things falling down" (so to speak).

All theories in physics (and in the other sciences too, for that matter) are models. It is common for a model to provide a very good fit to observation in one regime only to fail in some other regime (high velocity, short distances, high energy, etc.)

General relativity and QM (more precisely, the Standard Model of particle physics) are both very good and highly precise models, but they are inconsistent. A QM description is in the form of a wave function that yields the probability of a particle being at different points; thus a particle may be in a superposition of being in two different places. GR has no mechanism for calculating the gravity of something that is not in a well-defined location.

That's my understanding, anyway, not that I was ever any good at GR.

 

[corvus_corax]

I apologize for my previous post, it was far to general

Personally I do not think it actually works that way directly but I'm no expert on the subject.

From what I understand I actually does work that way. IOW, spacetime actually does warp around mass bodies.

All calculations using the model however are extremely accurate but I'm more inclined to think it's actually some sort of 'pulling towards force' rather than a 'slope for things to fall down'.

The most direct term I've read about (as a simple layman) is "path of least resistance" as opposed to "pulling"

It's kinda like explaining gravity with gravity because why would those objects fall down the slope (downwards) in the first place... It makes sense in our human perspective though.

I guess that's my question....the warping of spacetime makes sense to some of us, but if gravitons are needed/expected/etc, then is Relativity incorrect regarding the simple act of a mass body warping spacetime?

So as to your "why does a warping of the 4 dimensions "need" a "communicator" particle such as the graviton?" question. I think it's the other way around. The graviton particle is doing all the work and it needs a model/analogy of warping the 4 dimensions for us humans to understand the concept.

So you would posit that the concept of the warping of spacetime is actually incorrect, and only a model/analogy of what happens when (undemonstrated and hypothetical) gravitons "do what they do" (so to speak)?

Since the graviton praticle is still hypothetical I'm not sure if you can get a 100% clear answer but I too would love to hear more from someone with more knowledge about the subject.

Same here!
And where ARE our resident physicists (WiccanChild...Arunma....*ring ring ring* )

 

[LifeToTheFullest!]

SciAm just had a cover story (Feb 2009 "Was Einstein Wrong?") regarding GR/QM. I read it twice and still don't quite get it.

Seems there is conflicting details with GR & QM. ATM, it looks like QM better describes spacetime.

 

[sfs]

And where ARE our resident physicists (WiccanChild...Arunma....*ring ring ring* )

I'm a resident physicist, and I already answered.

 

[corvus_corax]

I'm a resident physicist

My apologies, I didn't realize that you had your degrees

and I already answered.

As I read it and (possibly mis-) understood it, you didn't answer the question directly.
I know they are inconsistent.
I know that they do not agree (and when agreement is attempted, idiotic results come of them).

thus a particle may be in a superposition of being in two different places.

I know that as well. As a matter of fact it may be in a superposition of being in more than two different places.
I still don't understand how this answers my OP.

Perhaps I just need more clarification from you? (seriously, if you're a physicist, I'm willing to listen and, more importantly, LEARN)

This is not a "challenge/debate" thread. It's just a thread borne from a question a layman has.

 

[sfs]

My apologies, I didn't realize that you had your degrees

Due to the odd ways of Yale University, I actually have four degrees in physics. That strikes me as rather excessive, given that I'm not a physicist anymore.

As I read it and (possibly mis-) understood it, you didn't answer the question directly.

No, I didn't really answer it, did I?

I believe the answer is that GR will turn out to be only an approximately correct model, and that its treatment of gravity as a warping of space-time will break down at high precision, at short distances and at high energies. In its place there will have to be a quantum gravity. QM will also have to change, since there is currently no working quantum mechanical theory of gravity. So both existing theories are likely wrong.

What I was trying to explain was one reason why gravity has to be treated by a quantum theory. It's hard to imagine how we could get rid of the quantum aspects of particle physics (although they may well turn out to be mere approximations to some deeper theory), and therefore the gravitational behavior of particles will also need to be quantized.

Also, the existence of singularities in GR is a a strong hint that the theory is breaking down in the extreme limit.

 

[Wiccan_Child]

All theories in physics (and in the other sciences too, for that matter) are models. It is common for a model to provide a very good fit to observation in one regime only to fail in some other regime (high velocity, short distances, high energy, etc.)

General relativity and QM (more precisely, the Standard Model of particle physics) are both very good and highly precise models, but they are inconsistent. A QM description is in the form of a wave function that yields the probability of a particle being at different points; thus a particle may be in a superposition of being in two different places. GR has no mechanism for calculating the gravity of something that is not in a well-defined location.

That's my understanding, anyway, not that I was ever any good at GR.

Haha you beat me to it! GR and QM very good in their own domains, but some circumstances can be described by both theories (e.g., a black hole's singularity: it's tiny, so QM, and highly gravimetric, so GR). But QM is more accurate, so physicsts (myself included) like to reject GR in its favour.

Poor old Einstein.

Anyway, to answer the OP, the graviton is the quantum mechanical way of describing gravity, and only exists by way of analogy with the other three forces and their mediators (the photon, gluon, and W & Z bosons).
Personally, I think gravity is a special force, in that it moves spacetime instead of particles.

Due to the odd ways of Yale University, I actually have four degrees in physics. That strikes me as rather excessive, given that I'm not a physicist anymore.

I'll take one if there's any going spare .

 

[arunma]

Hi Corvus, sorry I'm late.

I've often wondered about this myself, and then last summer another grad student explained it in a way that made perfect sense to me. If you're asking what I think you're asking, then the answer is that the rubber sheet and graviton models are actually identical. The motion of massive objects in space causes gravitational waves. As we know from the wave-particle duality and the quantum mechanical treatmen of matter, particles can be equivalently represented as waves, and vice versa. When a wave is represented by a particle, that particle is called a gauge boson. So gravitational waves can be represented by a gauge boson, which we call the graviton. This actually isn't all that strange. For example, sound vibrations in crystal lattices are reprsented by particles called phonons. The graviton is just the gravitational equivalent of this. So, to answer your questions.

1.) Depends on your definition of necessary. If it turns out the graviton doesn't exist, then the universe seems to not need it in order to continue existing (unless of course the universe decides to disappear in a poof of logic). However, it would cause the standard model to make a lot less sense sense. Electromagnetism, the strong force, and the weak force are all mediated by gauge bosons. If gravity alone wasn't, then we'd have to wonder why it's so different from the other forces.

2.) No, the graviton isn't contrary to the theory of general relativity. As I mentioned above, the gauge boson and wave representations are equally valid. A gravity wave can be equivalently represented by a particle, just like light.

3.) It's important to keep in mind that we already know that general relativity is incorrect, because it contradicts quantum mechanics. QM is one of the most tested and verified theory in all of science. It has an elegant formalism, correctly predicts a variety of physical effects, and seems to be incapable of goofing (so far). Since it contradicts GR, we summarily conclude that GR is wrong. Why physicists don't go the other way, I'm not really sure. Perhaps it's because QM is much easier to test than GR. All you have to do to test QM is wander over to a condensed matter lab or check out the LHC. I guess GR is "out there," and it's more difficult to get observations. Whatever the case, GR is widely considered to be a faulty theory, so whatever happens to the graviton, we know we're going to need a better model someday.

Anyway, I hope this helps. Fortunately Sfs is here to correct me if something I've said is wrong.